I'm in the middle of doing some research for some papers and
have been glancing through "Entering Space" by Robert Zubrin.

(As you may remember, I gave his "Case for Mars" a hearty
thumbs up.)

I'm only skimming chapters, but extracts include:

Re: Nanotech

> "This could lead to the manifestation of systems that would have all
> of the capabilities of the evil "liquid metal" robot depicted in the
> movie Terminator 2, ..."

> "Furthermore, while nanotechnology may not violate any laws of physics,
> *controllable* self-replicating robots may well violate the laws of
> *biology*. Consider that small replicating micromachines will
> unquestionably undergo random alterations, or mutations if you will.
> Those mutations that produce strains that reproduce more rapidly will
> swiftly outnumber to insignificance those that don't."

Huh? If humans allow the creation of nanotech with insufficient
error-correction-codes (ECC) in designs/programs to allow this to
occur, then they get what they deserve.

> "... another reason to hold nanorobots suspect--we don't observe
> them. If diamond-geared, self-replicating assemblers could be built,
> they would be ideally suited for dispersal across interstellar space
> using microscopic solar sails for propulsion. If, in the vast sweep
> of past time, a single species anywhere in the Milky Way developed
> such microautomatons, it long since would have been able to use them
> to colonize the entire galaxy."

Blech, Phooey. All you have to have done is existed for a few million
years longer than humans and developed slow-pokey world ships to have
colonized the galaxy. It doesn't take nanotech. Furthermore, the
"nanobot" colonization perspective indicates that he clearly does not
understand the possible problems that nanomachinery has with the high
radiation fluxes encountered in space.

> "... we are driven toward concluding that (a) there is no other
> intelligent life in the galaxy or (b) non-organic nanotechnology
> of the self-replicating micro-Babbage-robot type described by Drexler
> is impossible. Since we know that the evolution of intelligent life
> is posssible, but we do not know that nanotechnology is, I must consider
> (b) the more likely alternative.

Great! Using the Fermi Paradox to explain why nanotechnology is
impossible. Now I've seen everything! Could there not be a (c)
"Nanotech civilizations can observe almost everything, so there is
no need to travel", or (d) "Nanotech civilizations spend their time
trying to engineer smaller selves. They do not interact with
pre-limts-of-physics species." etc.

> "But it's worth some speculation, because if the promise ever does
> pan out, programmable self-replicating nanomachines will offer our
> descendents powers of creation limited only by the rate at which
> solar flux provides the energy needed to drive work in a given region.

Oooops.... Zubrin needs to talk to Von Err about the prospects for
diamondoid nanoassembly before "our descendents" are born. Further
he clearly doesn't understand that the limit may not be "power in"
(since you can clearly have diamondoid fusion reactors), but "waste heat
out". The limits are fundamentally how efficient you can make your
construction or thought operations and how effectively you can get rid
of the heat generated.

Re: Dyson shells
> "To date, no Dyson spheres have been observed."

(Because the searches are primarily still looking at stars!!! Duh!)

> "The 1-AU sphere described in the previous paragraph, if given a shell
> just 1 meter thick, would require the mass from 260 disassembled Earths
> to build (and 1 meter is much too thin for it to hold together). This
> much solid material is unavailable in the solar system (Jupiter's hydrogen
> would be useless for construction), and is unlikely to be available to
> others. ... the gravity vector almost everywhere on its surface would
> point in rather inconvenient directions..."

Sigh! Zubrin (and his reviewers who include a "Who's Who" of aeronautics)
clearly haven't read Dyson's responses to letters to his original article.
Dyson clearly states that he is not suggesting a "solid sphere". (That is
why it is better to call them Dyson "shells".) Russian papers during the
1960's and 1970's (presumably also not reading the letters), e.g.
1. "Dysonís Sphere Is Impossible", V. D. Davydov,
Priroda, XX 64/4 N 11 XI (1963), p.100-101
2. Searching for Extraterrestrial Civilizations", G. I. Pokrovskii,
Priroda XX 64/4, N 6:97-98 (1973)
proved that solid spheres were impossible. However, nested layers of
orbiting solar power collectors with computers are not impossible.

All of Zubrin's subsequent discussion of dismantling Venus to
construct a Ringworld or building masses of O'Neill colonies
is irrelevant because it assumes Dyson shells are impossible.

So, it would appear that Zubrin's perspective argues that only
"humans" in their natural form will be the logical explorers
of the solar system.